The proposed research concerns the development of photoaffinity labels for phosphate derivative binding sites. It will involve the investigation of a new type of photoaffinity label based on phosphono-stabilized diazo compounds. A series of compounds incorporating the diazophosphonoacetamide structure will be synthesized and used as photolabels for the catalytic site of aspartate transcarbamylase, and diazomethyl or -methylene analogs of cytosine triphosphate will be prepared and used as photolabels for the regulatory chain of the same enzyme. These systems will allow compounds of this type to be evaluated as photoaffinity labels, and will enable the technique to be extended to many other phosphate derivative binding sites. An improved photolysis apparatus for photolabeling experiments is also proposed.